The electrical power system (grid) includes generating sites, transmission lines, distribution equipment and end uses (loads). The electricity is generated competitively at multiple power plants and transmitted from the generating sites over the transmission lines regulated by the Federal Energy Regulatory Commission (FERC). Generation and transmission of the electricity are usually managed and controlled by regional entities that monitor generating capacity, real-time generation and loads, grid operations, market operations (buying and selling of electricity and delivery of the electricity to the buying party), system security and other aspects of the electric power system. There are a number of organizations responsible for overseeing power generation, transmission and distribution activities. Such controlling organizations include regional independent system operators (ISOs), regional transmission organizations (RTOs), reliability coordinators and utility companies. The transmission lines, which may be located in different states, are typically owned by an electric utility or a transmission company.
For safe and efficient operation of the power system, the generated electricity must instantaneously and continuously match the electrical load (i.e., the consumption and generation must be held in balance). Further the grid alternating current frequency (and thus the frequency of each generating unit) must be maintained within a very narrow range around 60 Hz. Excess generation causes the system frequency to increase while deficit generation causes the frequency to decrease. Although it is impossible to maintain a perfect generation and load balance, active control systems attempt to do this by constantly adjusting the power output of the generators. In addition to load imbalance, system frequency deviations from 60 Hz may also be caused by generators that are not properly meeting their desired generation targets, leading to over-generation or under-generation.
Small frequency deviations (e.g., less than about ±0.05 Hz) do not degrade system reliability or market efficiency (the buying and selling of electricity and transmission of the electricity over transmission lines connecting the generator to the load). Large frequency deviations from the nominal 60 Hz (e.g., 0.1 Hz), such as caused by the sudden loss of a generator can damage generation, transmission and load equipment, degrade product quality (causing lights to flicker, for example), collapse the power system (by triggering protective system actions, for example) and overload transmission lines as the remaining generators supply additional electricity to those lines in an attempt to restore the system frequency. Although the power system is designed to quickly recover from the loss of a generator, recovery typically takes several minutes. For example, the loss of a 2600 MW generator in a relatively small grid causes the frequency to drop by about 0.08 Hz and recover in about 10 minutes. Recovery is typically accomplished by tripping generators off line to overcome an over-frequency condition and shedding load to overcome an under frequency condition
The power system is divided into control areas (there are approximately 153 control areas (CAs) in the United States) with each control area exercising some control over the system frequency. An inter-area control system, referred to as an area generation control system ensures that all areas cooperate to control the system frequency. An area control error (ACE), computed for each area within the AGC system indicates the amount of frequency control coordination that a control area is required to contribute to the inter-area system. Use of the ACE spreads the economic burden of frequency control to all participating generators in all control areas. System reliability is thereby enhanced by avoiding reliance on a few generators for system frequency control, improving the system's ability to respond to transient conditions. The ACE value provides a technique to balance frequency maintenance across the entire system.
A frequency bias term is added to the ACE balancing equation to refine frequency control. The nominal target frequency is periodically increased or decreased by about 0.02 Hz to adjust the long term average frequency. Under normal operating conditions the frequency is controlled very tightly.
Balancing aggregate load with aggregate generation in a control area is accomplished through several services that are distinguished by the time frame over which they operate. Under normal operating conditions (i.e., no system disturbances) these services include regulation and load following. Regulation uses on-line generation capacity or stored capacity that is equipped with automatic generation control (AGC) that can change output quickly to compensate for minute-to-minute fluctuations in area load and unintended fluctuations in generation. A typical large fossil-fired plant thermal generator may be able to ramp 1% of its capacity in one minute. Smaller units can and combustion turbines can ramp faster. Loads are controlled by a load shedding function that can shed or restore loads as desired.
Load following uses on-line generation, stored capacity or load shedding equipment to compensate for the inter-hour and intra-hour load ramping. The regulation and load following differ only in the time period over which they operate.
Control area operators are not required to specifically procure load following generation. Instead, the required generation is procured in the short term energy market responsive to the real time energy prices and expected loads. Regulation, however, requires faster response than obtainable from units responding solely to market conditions, Instead, generators (and potentially energy storage units) offer capacity that can be controlled by the system operator's AGC system to balance the system's generation and load.
The control area operator is responsible for controlling its generating units. The AGC system calculates the control command for each generating unit (that is, each generating unit that is under AGC control) and issues the control signal to the generating unit on a per AGC control cycle basis. The AGC control cycle is the rate at which control signals are issued to the generating units. Typically the control cycle is four seconds, but it can be as short as two seconds or as long as five seconds. Thus the AGC system regulates the power output of the control area electric generators in response to changes in system frequency, loading on the interconnected system and the relation of these two parameters. The AGC system attempts to maintain the scheduled system frequency and established power interchange with other control areas, within predetermined limits. The AGC system monitors and controls power generation with the objectives of minimizing ACE, minimizing operating costs, maintaining generation at fixed (base load) values, maintaining net interchange power to the scheduled interchange power, maintaining actual system frequency at the scheduled frequency, and providing for ramp generation in a linear fashion according as scheduled.
Each control area is not able and not required to perfectly match generation and load. Generating an amount of electricity that is in exact equilibrium with the load is extremely difficult and impractical. Instead, control area operators strive to continually alternate between over and under generation. For example, a control area may impose a target of crossing the break even point (i.e., a zero ACE value) fifteen times per hour.
The North American Electric Reliability Council (NERC) has established rules governing how well each control area must balance load and generation. Control performance standards 1 and 2 (CPS 1 and 2) establish statistical limits on how well each control area must balance minute-to-minute fluctuations without degrading system reliability, where the system comprises multiple interconnected control areas. Since a balanced total system is desired, when one control area fails to balance its load and generation, generation in another control area provides the required balancing energy.
CPS1 and CPS2 are standards that measure overall control area performance. CPS1 measures the relationship between the control area's ACE and the system (i.e., interconnected control areas) frequency on a one-minute average basis for the previous eleven months back from the current minute (i.e., a rolling average). When the current minute is the last minute of the calendar month, the CPS1 indicates the relationship for the previous eleven months plus the current month now ended. The twelve month moving window period includes the current month (to the last day and minute of the current month) and the previous eleven months. Each month refers to an entire calendar month, that is, from the first day of a month to the last day of the same month. For example, if the current date is Mar. 5, 2007, the twelve month period runs from Apr. 1, 2006 00:00:00 to Mar. 31, 2007 23:59:59.
CPS1 represents a correlation of the clock-minute frequency deviation average (where clock-minute average refers to the average of all the instantaneous values (e.g., one measured or telemetered instantaneous value every four seconds) during a clock minute) and the clock-minute ACE average over a rolling 12 month period. When the system frequency is above its reference, under generation benefits the system (interconnected control areas) by lowering the system frequency and improves the CPS1 value. Over generation at such times, however, tends to worsen the CPS1 value. Thus CPS1 distinguishes between generation/load imbalances that help to restore the system frequency (a favorable CPS1 value) and those that degrade the system frequency (an unfavorable CPS1 value). The component parameters of CPS1 are determined every minute but the CPS1 values is evaluated and reported on a 12 month rolling average basis. NERC regulations require that each control area must be no less than 100% compliant with CPS1.
CPS2, a monthly performance standard, sets specific control area limits on the maximum ACE for every real time (clock time) 10 minute period. Control areas are permitted to exceed the CPS2 limit no more than 10% of the time, that is, a 90% compliance with the CPS2 value is required. Thus a control area can have no more than an average of about 14.4 CPS2 violations per day during each month. The CPS2 reference to the current month means a calendar month, Mar. 1, 2007 00:00:00 to Mar. 31, 2007 23:59:59.
There is a need for a method and system that reduces the risk of non-compliance with the CPS1 and CPS2 control performance standards set by regulatory authorities such as NERC.